Level 2: Identical to level 1 only faster. The array with sorted keys and hash lookup table are only recreated when a new key is added, a value is changed, or the sort routine is changed.

That's not necessarely faster. It depends on how often you are inserting and how often you are asking for the keys. What you could do is some form of hybrid:

• Create an array @sorted, and a variable $clean, initially set to 0.
• If FIRSTKEY is called, check the value of $clean. If it's true, @sorted contains the sorted keys. If false, fill @sorted with the sorted keys of the hash, and set $clean to 1.
• If you insert a key in the hash, or change the sort order, or delete something from the hash, set $clean to 0.
• This might cause some bizarre results if you modify a hash while iterating over it, but that may happen with normal hashes too.

Deleting a key doesn't require a rebuild because the element is deleted from the array and the lookup hash.

Are you aware that deleting an element from an array can take time linear in the size of the array?

Abigail